1. Field of the Invention
The present invention relates to a thermal recording technique such as a thermal paper recording technique and a thermal transfer recording technique and, more particularly, to a method and an apparatus for thermal recording suitable for recording (e.g., printing and plotting) image data on a large-sized recording sheet.
2. Discussion of the Background
An automatic plotter is required to draw figures on a large-sized recording sheet such as A0- and A1-sized sheets. Conventionally, a thermal recording apparatus using an in-line type thermal head as well as a so-called pen plotter has been used in such an automatic plotter or a similar apparatus. The thermal recording apparatus using the in-line type thermal head has the advantage of high-speed operation, low noise, easy maintenance and the like. It is desirable that a long-sized thermal head adaptable for the A0- and A1-sized sheets is used in the thermal recording apparatus. Since, however, the long-sized thermal head is low in yield, high in manufacturing cost, and difficult in maintenance, it cannot be put to practical use. In a thermal recording apparatus for thermally recording image data on a large-sized recording sheet, therefore, a plurality of popular thermal heads such as thermal heads for A3-sized sheets are arranged along a line, i.e., in a line direction to perform a recording operation on large-sized sheets such as A0- and A1-sized sheets.
For example, as shown in FIG. 1, in a thermal recording apparatus capable of recording image data on a A0-sized recording sheet, three thermal heads 1-3 are arranged in parallel and in a staggered fashion, and thermal head 2 between thermal heads 1 and 3 is arranged ahead of thermal heads 1 and 3 in a direction in which recording sheets are fed. These three thermal heads are in-line type thermal heads adaptable for A3-sized recording sheets each having a resistance heating element array in which a number of resistance heating elements are arranged in line. Thermal heads 1-3 are so arranged that their effective recording regions are in contact with each other in the line direction, i.e., in the axial direction of the thermal heads. Thermal heads 1-3 are provided with platen rollers 4-6, respectively. Recording sheet 7 is inserted and pressed between thermal heads 1-3 and platen rollers 4-6. While recording sheet 7 is moving in the direction of arrow F shown in FIG. 1, the resistance heating elements of thermal heads 1-3 are selectively turned on, thereby recording image data on sheet 7. The image data is recorded on regions A, B and C of recording sheet 7, which are arranged in the line direction, by thermal heads 1, 2 and 3, respectively.
However, in the conventional thermal recording apparatus described above, if a gap occurs between the recording regions corresponding to thermal heads 1 to 3 shown in FIG. 1, the gap appears to be a blur on a recording sheet, even though it is a slight one. Therefore, the recording quality is remarkably lowered. It is thus necessary to precisely position the contact portions of the thermal heads, those contact portions corresponding to border portions of regions A, B and C; accordingly, the positioning operation is very complicated in manufacturing the thermal recording apparatus.